1. Field of the Invention
The present invention is directed to circuitry for on-chip protection against power supply load dump transients, to the structure of the devices utilized in such circuitry, and to methods for fabricating these structures.
2. Discussion of the Prior Art
Integrated circuit devices are easily damaged by power supply transients unless protection circuitry is provided. One of the most severe conditions to be protected against is known as "automotive load dump". This condition can be described in conjunction with the graph provided as FIG. 1.
As is well known, in the basic configuration of an automotive electrical system, a battery and an alternator are connected in parallel to the input V.sub.DD power supply pad of the various operative low current logic circuits in the system. The alternator is capable of generating a large current to charge the battery and to supply the other automotive loads. A "load dump" event occurs when the alternator's load is disconnected, for example when a battery connector comes loose or, in a less severe example, when the automobile's headlights are turned off. Because of the current flow through the alternator and its inherent inductance, a magnetic field is established which prevents the alternator's output current from changing rapidly. When the alternator's load is suddenly reduced, charge, and thus voltage, builds up at the V.sub.DD power supply pad of the operative circuits, which remain connected to the alternator's output.
As illustrated in FIG. 1, a sudden voltage increase, or "load dump transient", is applied to the VV.sub.DD supply pad at the time of the upset, t=0. This transient, which can be expressed as EQU V.sub.DD =14+46e.sup.-t/ 120 msec.
for the typical automotive system, is quite high close to the battery and decreases with distance from the battery. For example, the transient voltage might reach 80V in the engine compartment, 60V along the firewall and 40V in the passenger compartment.
The logic circuits in the system must, obviously, remain in their normal operating mode up to a V.sub.DD supply voltage of 16V. However, if the V.sub.DD supply voltage rises above 16V, such as in the event of a load dump transient, disruption of the circuit's normal operation is permitted, but no permanent damage can result within the circuit and it must not pass greater than 20V to other circuits in the system via its outputs. When, as shown in FIG. 1, the V.sub.DD supply voltage returns to less than 16V with time, the circuit must return to normal operation.
The conventional approach to solving the problem of load dump transients has been to provide an arrangement of external passive components between the power supply and the supply pad of the operative circuit. This typically takes the form of an external network of resistors and capacitors which serves as a filter for the voltage surge. Alternatively, an external inductor is inserted in series with the operative circuit to resist the changing current caused by the transient. These solutions require extra components, which increase cost, and consume valuable space within the system.